The present method and apparatus for electromagnetic heat treatment relates to the fracturing of a subsurface rock formations to access oil deposits and the heating of subsurface geological formations using radio frequency (“RF”) energy to assist in the production of oil from those deposits. In particular, the present invention relates to a method for using RF energy to facilitate the production of oil from formations separated from other formations by a rock stratum.
Bituminous ore, oil sands, tar sands, and heavy oil are typically found as naturally occurring mixtures of sand or clay and dense and viscous petroleum. Recently, due to depletion of the world's oil reserves, higher oil prices, and increases in demand, efforts have been made to extract and refine these types of petroleum ore as an alternative petroleum source. Because of the extremely high viscosity of bituminous ore, oil sands, oil shale, tar sands, and heavy oil, however, the drilling and refinement methods used in extracting standard crude oil are typically not available. Therefore, bituminous ore, oil sands, oil shale, tar sands, and heavy oil are typically extracted by strip mining, or in situ techniques are used to reduce the viscosity by injecting steam or solvents in a well so that the material can be pumped. Under either approach, however, the material extracted from these deposits can be a viscous, solid or semisolid form that does not easily flow at normal oil pipeline temperatures, making it difficult to transport to market and expensive to process into gasoline, diesel fuel, and other products. Typically, the material is prepared for transport by adding hot water and caustic soda (NaOH) to the sand, which produces a slurry that can be piped to the extraction plant, where it is agitated and crude bitumen oil froth is skimmed from the top. In addition, the material is typically processed with heat to separate oil sands, oil shale, tar sands, or heavy oil into more viscous bitumen crude oil, and to distill, crack, or refine the bitumen crude oil into usable petroleum products.
Steam is typically used to provide this heat in what is known as a steam assisted gravity drainage system, or SAGD system. Electric heating has also been employed. Such conventional methods of heating bituminous ore, oil sands, tar sands, and heavy oil suffer from numerous drawbacks. For example, the conventional methods typically utilize large amounts of water, and also large amounts of energy. Moreover, using conventional methods, it has been difficult to achieve uniform and rapid heating, which has limited successful processing of bituminous ore, oil sands, oil shale, tar sands, and heavy oil. SAGD systems may not be practical: (1) where there is insufficient caprock to contain the steam; (2) in permafrost regions; or (3) where the steam may be lost to thief zones. Conductive heating may be required to initiate the fluid movement to convect the steam, yet conductive heating is slow and unreliable such that may SAGD wells do not start. It can be desirable, both for environmental reasons and efficiency/cost reasons to reduce or eliminate the amount of water used in processing bituminous ore, oil sands, oil shale, tar sands, and heavy oil, and also provide a method of heating that is efficient and environmentally friendly, which is suitable for post-excavation processing of the bitumen, oil sands, oil shale, tar sands, and heavy oil. The heating and processing can take place in-situ, or in another location after strip mining the deposits.
RF heating many offers advantages over the above-described methods when heating bitumen. RF energy can be targeted, and reduces or eliminates the large amounts of water used in many other methods. Unlike steam, RF heating does not require convection to convey the heat energy. Thus, startup is reliable.
Antennas used for prior RF heating of heavy oil in subsurface formations have typically been dipole antennas. U.S. Pat. Nos. 4,140,179 and 4,508,168 disclose prior dipole antennas positioned within subsurface heavy oil deposits to heat those deposits. Arrays of dipole antennas have also been used to heat subsurface formations. U.S. Pat. No. 4,196,329 discloses an array of dipole antennas that are driven out of phase to heat a subsurface formation.
RF energy has been used to heat oil shale with the goal of producing gas and shale oil from kerogen contained in the shale. U.S. Pat. No. 4,193,451 discloses subjecting a body of oil shale to RF in the form of alternating electric fields having frequencies in the range of 100 kilohertz to 100 megahertz to produce controlled heating of kerogen in the oil shale. This heating may produce fissures in the oil shale, however, U.S. Pat. No. 4,485,869 discloses that those fissures are undesirable and teaches heating the oil shale relatively slowly to produce relatively little cracking of the oil shale.
Underground permeation is often inadequate in oil sand formations, largely due to the presence of rock strata in the formations. Often comprised of shale, these rock strata can impede the production of hydrocarbons from oil bearing formations when using traditional processing methods, such as SAGD systems. Such split pay zones are a large problem in the Athabasca oil sands. Shale in underground formations is a porous rock that typically contains internal water content and is characterized by thin laminae internally. In processing non-oil sand formations, rock strata is sometimes fractured using hydrofracturing, chemicals, or explosives. However, these methods of fracturing rock strata are not well suited to the recovery of oil from oil sands because, respectively, they require an on-site water source where there may be none, they require dangerous and expensive chemicals, or the thin, oil-bearing ore in these deposits may be damaged by the explosives used to fracture the rock strata present in the formation.